1. Field of the Invention
The present invention relates to a panel assembly structure in which electrode terminals of a panel are connected with electrode terminals of a flexible printed circuit board via an anisotropic conductive film and method for assembling the panel, and in particular, to a display panel assembly structure employing an electroluminescence device, plasma, liquid crystals, or the like.
2. Description of the Prior Art
Conventionally, electrode terminals of a panel have been connected with electrode terminals of a flexible printed circuit board in a manner as follows.
Referring to FIG. 20 which is a sectional view of a panel and a flexible printed circuit board and FIG. 21 (a sectional view taken along the line 21--21 in FIG. 20), electrode terminals 4 on a flexible printed circuit board 2 mounted with a drive IC (integrated circuit) 1 have been connected with electrode terminals 7 on a panel 8 by a thermocompression bonding method with interposition of an anisotropic conductive film 6 which has a film thickness b (20.times.10.sup.-3 mm to 30.times.10.sup.-3 mm) much greater than a diameter d (8.times.10.sup.-3 mm to 10.times.10.sup.-3 mm) of a conductive particle 5 (i.e., b&gt;&gt;d) and includes several conductive particles 5 in the direction of the thickness.
The electrode terminals 4 on the flexible printed circuit board 2 are formed to have a thickness a (=18.times.10.sup.-3 mm or more) by sticking a copper sheet onto the flexible printed circuit board 2 via a reinforcing adhesive sheet 3 and etching the copper sheet.
The flexible printed circuit board 2 having the fine-pitch electrode terminals is connected with the panel 8 having the fine-pitch electrode terminals, and then the conductive particles 5 in the anisotropic conductive film 6 are peripherally coated with insulating films 9. Then the insulating films 9 existing in the direction of the thickness of the anisotropic conductive film 6 (in the direction in which the electrode terminals are to be connected with each other) are peeled off by a pressure force exerted in connecting the electrode terminals 4 on the side of the flexible printed circuit board 2 with the electrode terminals 7 on the side of the panel 8 in the thermocompression bonding process, with which both the electrode terminals 4 and 7 are electrically connected with each other.
The following two factors can be enumerated as factors required in connecting the fine-pitch electrode terminals with each other.
(1) An improvement in etching accuracy of the top surfaces of the electrode terminals 4 in forming the electrode terminals 4 by etching on the flexible printed circuit board 2 to be mounted with the drive IC 1.
(2) Ensuring of a required amount of conductive particles 5 existing between each electrode terminal 7 on the side of the panel 8 and each electrode terminal 4 on the side of the flexible printed circuit board 2 in the thermocompression bonding process.
The following provides a review on the conventional method for connecting the panel 8 having the electrode terminals 7 with the flexible printed circuit board 2 having the electrode terminals 4.
Firstly, the etching accuracy of the top surface of the electrode terminal 4 deteriorates as the thickness a of the electrode terminal 4 to be etched increases. Therefore, in order to improve the etching accuracy, the thickness a of the electrode terminal 4 is required to be reduced. However, if the thickness a of the electrode terminal 4 is merely reduced, there is such a problem that the electrode terminal 4 may be bent or made to be easily broken by a stress applied in subsequently mounting the drive IC 1.
Secondly, the amount of the conductive particles 5 existing between both the electrode terminals 4 and 7 in the thermocompression bonding process depends on the rate of the existence of the conductive particles 5 between both the electrode terminals 4 and 7 just at the time when the conductive particles 5 existing in the anisotropic conductive film 6 are bonded to the electrode terminals in the thermocompression bonding process. In order to ensure a reliable connection strength, the anisotropic conductive film 6 is required to be sufficiently filled in each space portion between adjoining electrode terminals 4 and 4 on the side of the flexible printed circuit board 2. Therefore, the anisotropic conductive film 6 is required to have a film thickness corresponding to the thickness a of the electrode terminals 4 on the side of the flexible printed circuit board 2.
However, since the thickness a of the electrode terminals 4 is much greater than the conductive particle diameter d, the thickness of the anisotropic conductive film 6 is compelled to be much greater than the conductive particle diameter d. As a result, a resin portion of the anisotropic conductive film 6 flows actively in the thermocompression bonding process, with which the conductive particles 5 flow due to a stress generated when the resin flows. Consequently, the amount of the conductive particles 5 between the electrode terminals 4 and 7 is reduced to fail in ensuring the required amount of the conductive particles.
Also, there is such a problem that the conductive particles 5 which have been flown in the space portion between adjoining electrode terminals 4 and 4 on the side of the flexible printed circuit board 2 become agglomerate to be a cause of a current leak between the adjoining electrode terminals 4 and 4.
For the above-mentioned reasons, according to the method for connecting the panel 8 having the conventional electrode terminals 7 with the flexible printed circuit board 2, it is difficult to achieve a connection between electrode terminals having a fine pitch of not greater than 80.times.10.sup.-3 mm.
In order to reduce the possible occurrence of current leak between adjoining electrode terminals 4 and 4, it can be considered to form an insulating film 9 around each conductive particle 5 in the anisotropic conductive film 6. However, in the above-mentioned case, the insulating film 9 is required to be peeled off by a pressure force in the thermocompression bonding process, and therefore the panel 8 may be destroyed by the pressure force in the process. Furthermore, it is possible that the insulating film 9 cannot be completely peeled off.
Furthermore, as shown in FIG. 22, there has been conventionally provided only an alignment mark 10 at an end portion of an array of the electrode terminals 4 on the flexible printed circuit board 2, and no special device has been provided to prevent the anisotropic conductive film 6 from flowing out in the thermocompression bonding process. Therefore, the anisotropic conductive film 6 flows out at the end portion of the flexible printed circuit board 2 in a manner as shown in FIG. 23 (corresponding to a section taken along the line 23--23 in FIG. 22), resulting in a degraded connection reliability.